I got my FM Butterfly brace installed and the car now reminds me of a Chevy truck, solid like a rock. ;)
Seriously though, chassis flex seems to be eliminated but I haven't crossed any RR tracks yet. We'll so on the commute to work tomorrow.
Install was a bit of a PITA. Similar to a roll bar installation. Lots of coaxing my frame rails (read beating with a hammer) and lots of dirt! My car is DIRTY underneath and I even de-greased and power-washed it first!
It definitely makes a significant improvement to the chassis on my NA and a worthy upgrade to consider. If you do upgrade, have Gary at TDR make his modifications to the butterfly section. It allows for easier removal of the center section if you have to do exhaust work.